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<tr class="t-nv"><td colspan="5"> <a href="function_declaration.html" title="c/language/function declaration"> function declaration</a> </td></tr>
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<p>A function definition associates the function body (a sequence of declarations and statements) with the function name and parameter list. Unlike <a href="function_declaration.html" title="c/language/function declaration">function declaration</a>, function definitions are allowed at file scope only (there are no nested functions).
</p><p>C supports two different forms of function definitions:
</p>
<table class="t-sdsc-begin">

<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <span class="t-spar">attr-spec-seq</span><span class="t-mark">(optional)</span> <span class="t-spar">specifiers-and-qualifiers</span> <span class="t-spar">parameter-list-declarator</span>  <span class="t-spar">function-body</span>
</td>
<td> (1)
</td>
<td class="t-sdsc-nopad">
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <span class="t-spar">specifiers-and-qualifiers</span> <span class="t-spar">identifier-list-declarator</span> <span class="t-spar">declaration-list</span> <span class="t-spar">function-body</span>
</td>
<td> (2)
</td>
<td> <span class="t-mark-rev t-until-c23">(until C23)</span>
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr></table>
<p>where
</p>
<table class="t-par-begin">


<tr class="t-par">
<td> <span class="t-spar">attr-spec-seq</span>
</td>
<td> -
</td>
<td>  <span class="t-mark-rev t-since-c23">(C23)</span>an optional list of <a href="attributes.html" title="c/language/attributes">attributes</a>, applied to the function
</td></tr>
<tr class="t-par">
<td> <span class="t-spar">specifiers-and-qualifiers</span>
</td>
<td> -
</td>
<td>  a combination of
<ul><li> <a href="declarations.html" title="c/language/declarations">type specifiers</a> that, possibly modified by the declarator, form the <i>return type</i>
</li><li> <a href="storage_duration.html" title="c/language/storage duration">storage class specifiers</a>, which determine the linkage of the identifier (<code>static</code>, <code>extern</code>, or none)
</li><li> function specifiers <a href="inline.html" title="c/language/inline"><code>inline</code></a>, <a href="_Noreturn.html" title="c/language/ Noreturn"><code>_Noreturn</code></a>, or none 
</li></ul>
</td></tr>
<tr class="t-par">
<td> <span class="t-spar">parameter-list-declarator</span>
</td>
<td> -
</td>
<td>  a declarator for a function type which uses a <a href="function_declaration.html" title="c/language/function declaration">parameter list</a> to designate function parameters
</td></tr>
<tr class="t-par">
<td> <span class="t-spar">identifier-list-declarator</span>
</td>
<td> -
</td>
<td>  a declarator for a function type which uses a <a href="function_declaration.html" title="c/language/function declaration">identifier list</a> to designate function parameters
</td></tr>
<tr class="t-par">
<td> <span class="t-spar">declaration-list</span>
</td>
<td> -
</td>
<td>  sequence of declarations that declare every identifier in <span class="t-spar">identifier-list-declarator</span>. These declarations cannot use initializers and the only <a href="storage_duration.html" title="c/language/storage duration">storage-class specifier</a> allowed is <span class="t-c"><span class="mw-geshi c source-c"><span class="kw4">register</span></span></span>.
</td></tr>
<tr class="t-par">
<td> <span class="t-spar">function-body</span>
</td>
<td> -
</td>
<td>  a <a href="statements.html#Compound_statements" title="c/language/statements">compound statement</a>, that is a brace-enclosed sequence of declarations and statements, that is executed whenever this function is called
</td></tr></table>
<div class="t-li1"><span class="t-li">1)</span> New-style (C89) function definition. This definition both introduces the function itself and serves as a function prototype for any future <a href="operator_other.html#Function_call" title="c/language/operator other">function call expressions</a>, forcing conversions from argument expressions to the declared parameter types.
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="c source-c"><pre class="de1"><span class="kw4">int</span> max<span class="br0">(</span><span class="kw4">int</span> a, <span class="kw4">int</span> b<span class="br0">)</span>
<span class="br0">{</span>
    <span class="kw1">return</span> a<span class="sy1">&gt;</span>b<span class="sy4">?</span>a<span class="sy4">:</span>b<span class="sy4">;</span>
<span class="br0">}</span>
 
<span class="kw4">double</span> g<span class="br0">(</span><span class="kw4">void</span><span class="br0">)</span>
<span class="br0">{</span>
    <span class="kw1">return</span> <span class="nu16">0.1</span><span class="sy4">;</span>
<span class="br0">}</span></pre></div></div></div>
<div class="t-li1"><span class="t-li">2)</span> <span class="t-mark-rev t-until-c23">(until C23)</span> Old-style (K&amp;R) function definition. This definition does not behave as a prototype and any future <a href="operator_other.html#Function_call" title="c/language/operator other">function call expressions</a> will perform default argument promotions.
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="c source-c"><pre class="de1"><span class="kw4">int</span> max<span class="br0">(</span>a, b<span class="br0">)</span>
<span class="kw4">int</span> a, b<span class="sy4">;</span>
<span class="br0">{</span>
    <span class="kw1">return</span> a<span class="sy1">&gt;</span>b<span class="sy4">?</span>a<span class="sy4">:</span>b<span class="sy4">;</span>
<span class="br0">}</span>
<span class="kw4">double</span> g<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
    <span class="kw1">return</span> <span class="nu16">0.1</span><span class="sy4">;</span>
<span class="br0">}</span></pre></div></div></div>
<h3><span class="mw-headline" id="Explanation">Explanation</span></h3>
<p>As with <a href="function_declaration.html" title="c/language/function declaration">function declarations</a>, the return type of the function, determined by the type specifier in <span class="t-spar">specifiers-and-qualifiers</span> and possibly modified by the <span class="t-spar">declarator</span> as usual in <a href="declarations.html" title="c/language/declarations">declarations</a>, must be a complete non-array object type or the type <span class="t-c"><span class="mw-geshi c source-c"><span class="kw4">void</span></span></span>. If the return type would be cvr-qualified, it is adjusted to its unqualified version for the purpose of constructing the function type.
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="c source-c"><pre class="de1"><span class="kw4">void</span> f<span class="br0">(</span><span class="kw4">char</span> <span class="sy2">*</span>s<span class="br0">)</span> <span class="br0">{</span> <a href="../io/puts.html"><span class="kw836">puts</span></a><span class="br0">(</span>s<span class="br0">)</span><span class="sy4">;</span> <span class="br0">}</span> <span class="co1">// return type is void</span>
<span class="kw4">int</span> sum<span class="br0">(</span><span class="kw4">int</span> a, <span class="kw4">int</span> b<span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span> a<span class="sy2">+</span>b<span class="sy4">:</span> <span class="br0">}</span> <span class="co1">// return type is int</span>
<span class="kw4">int</span> <span class="br0">(</span><span class="sy2">*</span>foo<span class="br0">(</span><span class="kw4">const</span> <span class="kw4">void</span> <span class="sy2">*</span>p<span class="br0">)</span><span class="br0">)</span><span class="br0">[</span><span class="nu0">3</span><span class="br0">]</span> <span class="br0">{</span> <span class="co1">// return type is pointer to array of 3 int</span>
    <span class="kw1">return</span> <a href="../memory/malloc.html"><span class="kw403">malloc</span></a><span class="br0">(</span><span class="kw4">sizeof</span><span class="br0">(</span><span class="kw4">int</span><span class="br0">[</span><span class="nu0">3</span><span class="br0">]</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span></pre></div></div>
<p>As with <a href="function_declaration.html" title="c/language/function declaration">function declarations</a>, the types of the parameters are adjusted from functions to pointers and from arrays to pointers for the purpose of constructing the function type and the top-level cvr-qualifiers of all parameter types are ignored for the purpose of determining <a href="compatible_type.html#Compatible_types" title="c/language/type">compatible function type</a>.
</p>
 <table class="t-rev-begin">
<tr class="t-rev t-until-c23"><td>
<p>Unlike <a href="function_declaration.html" title="c/language/function declaration">function declarations</a>, unnamed formal parameters are not allowed (otherwise, there would be conflicts in old-style (K&amp;R) function definitions), they must be named even if they are not used within the function. The only exception is the special parameter list <span class="t-c"><span class="mw-geshi c source-c"><span class="br0">(</span><span class="kw4">void</span><span class="br0">)</span></span></span>.
</p>
</td>
<td><span class="t-mark-rev t-until-c23">(until C23)</span></td></tr>
<tr class="t-rev t-since-c23"><td>
<p>Formal parameters may be unnamed in function definitions, because old-style (K&amp;R) function definitions has been removed. Unnamed parameters are inaccessible by name within the function body.
</p>
</td>
<td><span class="t-mark-rev t-since-c23">(since C23)</span></td></tr>
</table>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="c source-c"><pre class="de1"><span class="kw4">int</span> f<span class="br0">(</span><span class="kw4">int</span>, <span class="kw4">int</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// declaration</span>
<span class="co1">// int f(int, int) { return 7; } // Error until C23, OK since C23</span>
<span class="kw4">int</span> f<span class="br0">(</span><span class="kw4">int</span> a, <span class="kw4">int</span> b<span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span> <span class="nu0">7</span><span class="sy4">;</span> <span class="br0">}</span> <span class="co1">// definition</span>
<span class="kw4">int</span> g<span class="br0">(</span><span class="kw4">void</span><span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span> <span class="nu0">8</span><span class="sy4">;</span> <span class="br0">}</span> <span class="co1">// OK: void doesn't declare a parameter</span></pre></div></div>
<p>Within the function body, every named parameter is an <a href="value_category.html" title="c/language/value category">lvalue</a> expression, they have automatic <a href="storage_duration.html" title="c/language/storage duration">storage duration</a> and <a href="scope.html" title="c/language/scope">block scope</a>. The layout of the parameters in memory (or if they are stored in memory at all) is unspecified: it is a part of the <a href="https://en.wikipedia.org/wiki/Calling_convention" class="extiw" title="enwiki:Calling convention">calling convention</a>.
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="c source-c"><pre class="de1"><span class="kw4">int</span> main<span class="br0">(</span><span class="kw4">int</span> ac, <span class="kw4">char</span> <span class="sy2">**</span>av<span class="br0">)</span>
<span class="br0">{</span>
    ac <span class="sy1">=</span> <span class="nu0">2</span><span class="sy4">;</span> <span class="co1">// parameters are lvalues</span>
    av <span class="sy1">=</span> <span class="br0">(</span><span class="kw4">char</span> <span class="sy2">*</span><span class="br0">[</span><span class="br0">]</span><span class="br0">)</span><span class="br0">{</span><span class="st0">"abc"</span>, <span class="st0">"def"</span>, <a href="../types/NULL.html"><span class="kw103">NULL</span></a><span class="br0">}</span><span class="sy4">;</span>
    f<span class="br0">(</span>ac, av<span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span></pre></div></div>
<p>See <a href="operator_other.html#Function_call" title="c/language/operator other">function call operator</a> for other details on the mechanics of a function call and <a href="return.html" title="c/language/return">return</a> for returning from functions.
</p>
 <table class="t-rev-begin">
<tr class="t-rev t-since-c99"><td>
<h3> <span class="mw-headline" id="func">__func__</span></h3>
<p>Within every <span class="t-spar">function-body</span>, the special predefined variable <span class="t-c"><span class="mw-geshi c source-c">__func__</span></span> with block scope and static storage duration is available,  as if defined immediately after the opening brace by
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="c source-c"><pre class="de1"><span class="kw4">static</span> <span class="kw4">const</span> <span class="kw4">char</span> __func__<span class="br0">[</span><span class="br0">]</span> <span class="sy1">=</span> <span class="st0">"function name"</span><span class="sy4">;</span></pre></div></div>
<p>This special identifier is sometimes used in combination with the <a href="../preprocessor/replace.html" title="c/preprocessor/replace">predefined macro constants</a> <span class="t-c"><span class="mw-geshi c source-c">__FILE__</span></span> and <span class="t-c"><span class="mw-geshi c source-c">__LINE__</span></span>, for example, by <span class="t-lc"><a href="../error/assert.html" title="c/error/assert">assert</a></span>.
</p>
</td>
<td><span class="t-mark-rev t-since-c99">(since C99)</span></td></tr>
</table>
<h3><span class="mw-headline" id="Notes">Notes</span></h3>
<p>The argument list must be explicitly present in the declarator, it cannot be inherited from a typedef
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="c source-c"><pre class="de1"><span class="kw1">typedef</span> <span class="kw4">int</span> p<span class="br0">(</span><span class="kw4">int</span> q, <span class="kw4">int</span> r<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// p is a function type int(int, int)</span>
p f <span class="br0">{</span> <span class="kw1">return</span> q <span class="sy2">+</span> r<span class="sy4">;</span> <span class="br0">}</span> <span class="co1">// Error</span></pre></div></div>
 <table class="t-rev-begin">
<tr class="t-rev t-until-c99"><td>
<p>In C89, <span class="t-spar">specifiers-and-qualifiers</span> was optional, and if omitted, the return type of the function defaulted to <span class="t-c"><span class="mw-geshi c source-c"><span class="kw4">int</span></span></span> (possibly amended by the <span class="t-spar">declarator</span>).
</p><p>In addition, old-style definition didn't require a declaration for every parameter in <span class="t-spar">declaration-list</span>. Any parameter whose declaration was missing had type <span class="t-c"><span class="mw-geshi c source-c"><span class="kw4">int</span></span></span>
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="c source-c"><pre class="de1">max<span class="br0">(</span>a, b<span class="br0">)</span> <span class="co1">// a and b have type int, return type is int</span>
<span class="br0">{</span>
    <span class="kw1">return</span> a<span class="sy1">&gt;</span>b<span class="sy4">?</span>a<span class="sy4">:</span>b<span class="sy4">;</span>
<span class="br0">}</span></pre></div></div>
</td>
<td><span class="t-mark-rev t-until-c99">(until C99)</span></td></tr>
</table>
<h3><span class="mw-headline" id="Defect_reports">Defect reports</span></h3>
<p>The following behavior-changing defect reports were applied retroactively to previously published C standards.
</p>
<table class="dsctable" style="font-size:0.8em">
<tr>
<th> DR
</th>
<th> Applied to
</th>
<th> Behavior as published
</th>
<th> Correct behavior
</th></tr>
<tr>
<td> <a rel="nofollow" class="external text" href="https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2396.htm#dr_423">DR 423</a>
</td>
<td> C89
</td>
<td> the return type might be qualified
</td>
<td> the return type is implicitly disqualified
</td></tr></table>
<h3><span class="mw-headline" id="References">References</span></h3>
<div class="t-ref-std-17">
<ul><li> C17 standard (ISO/IEC 9899:2018): 
</li></ul>
<dl><dd><ul><li> 6.9.1 Function definitions (p: 113-115)
</li></ul>
</dd></dl>
</div>
<div class="t-ref-std-11">
<ul><li> C11 standard (ISO/IEC 9899:2011): 
</li></ul>
<dl><dd><ul><li> 6.9.1 Function definitions (p: 156-158)
</li></ul>
</dd></dl>
</div>
<div class="t-ref-std-c99">
<ul><li> C99 standard (ISO/IEC 9899:1999): 
</li></ul>
<dl><dd><ul><li> 6.9.1 Function definitions (p: 141-143)
</li></ul>
</dd></dl>
</div>
<div class="t-ref-std-c89">
<ul><li> C89/C90 standard (ISO/IEC 9899:1990): 
</li></ul>
<dl><dd><ul><li> 3.7.1 Function definitions 
</li></ul>
</dd></dl>
</div>
<h3><span class="mw-headline" id="See_also">See also</span></h3>
<table class="t-dsc-begin">

</table>

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